Leviim Were the Smallest

כָּל־פְּקוּדֵי הַלְּוִיִם אֲשֶׁר פָּקַד מֹשֶׁה וְּאַהֲרֹן עַל־פִי יְּהוָּה לְּמִשְּפְּחֹתָּם כָּל־זָּכָּר מִבֶׁן־חֹדֶׁש וָּמַעְּלָּה שְּנַיִם וְּעֶׁשְּרִים אָּלֶׁף:

BeMidbar 3, 39: The total head-count of male Leviim whose family (lineage was verified) from the age of one month and upward, counted by Moshe and Aharon by G-d’s word was 22,000.

The count of the Leviim was from the age of a month and older. The rest of tribes were counted from the age of twenty. Yet, the Leviim had the smallest population of all the tribes.

Census from Parshas BeMidbar

• Yehudah 74,600
• Dan 64,700
• Shimeon 59,300
• Zevulun 57,400
• Yissachar 54,400
• Naftoli 53,400
• Reuven 46,500
• Gad 45,650
• Asher 41,500
• Efraim 40,500
• Benyamin 35,400
• Manasheh 32,200
• Leviim 22,000

Ten explanations for this phenomenon are offered:

1. Some of the Leviim had the responsibility to carry the holiest of the Mishkan vessels. If they were not treated with proper respect it would bring about their death. (Rashi on Bereshis 29,34) Some raise a question on this explanation, as the counting in our Parsha is before the tasks were allotted. However, we can say that because the future held this awesome responsibility, it was an indication that the Leviim were on an elevated level, closer to HaShem than any other tribe. Therefore, any other shortcoming in their spiritual responsibilities were also dealt with harshly, as is derived from Tehillim 50,3, "usvivov nisaroh m'ode."
2. The population explosion of the Bnei Yisrael in Egypt was nothing short of miraculous. The Torah says, "kaa'sher y'anu oso kein yirbeh," - as they pained the Bnei Yisrael, so did they multiply. The increase in the number of Bnei Yisrael was proportionate to their suffering. Since the tribe of Levi was not subjugated to slavery, as per M.R. Shmos 5,16, they did not multiply in a miraculous manner. (Ramban)
3. Our patriarch Yaakov expressed himself with negative terms towards Shimon and Levi (Bereshis 49:5,6,7). This caused a limitation in their growth. Shimon's limitation takes place at the later counting (Bemidbar 26,14). (Ramban)
4. The Leviim were the spiritual leaders of the Bnei Yisrael from the earliest days in Egypt. Their total immersion in Torah weakened them physically, "Torah ma'teshes kocho shel odom" (Gemara Sanhedrin 26b). Because they were physically weak they reproduced in a limited manner. (Tzror Hamor)
5. The need for the Bnei Yisrael to be a large population was so that when they would enter Eretz Yisroel they would occupy a large area so that they would not be overpowered by wild animals, "pen tirbeh o'lecho chayas hasadeh" (Devarim 7,22). Since the Leviim would not inherit a large parcel of land and in the main would reside close to the Beis HaMikdash, there was no need to have a large number of Leviim. (Abarbanel)
6. Since the Leviim were to reside around the Mishkan compound, they were similar to the angels that descended onto Har Sinai at the time of the giving of the Torah. (This is well understood according to the explanation of the Ibn Ezra on Shmos 19,13 that the sanctity of Har Sinai was transferred to the Mishkan.) Since the number of angels who descended onto Har Sinai was 22,000, as per the verse in Tehilim 68:18, "Rechev Elokim ribosayim alfei shinon," so too, the number of Leviim in the desert was 22,000. (Kabbalistic writings)
7. The Gemara Sotah 12a relates that Amram, the recognized leader of the Bnei Yisrael divorced his wife rather than reproduce and have his children thrown into the river. All others followed suit and did the same. "All others" means all from the tribe of Levi only, as they did not have the slave mentality. They were not ready to attempt to evade the Egyptians by giving birth in the fields and leaving their children there, as did the rest of the Bnei Yisrael. They remained divorced from their wives for many years and therefore had a limited population. Amram took back his wife shortly after he divorced her because of the prophecy of his daughter Miriam, as related in the above-mentioned Gemara. (Ohr Hachaim Hakodosh and Kli Yakar)
8. If the Leviim were to multiply as greatly as the other tribes, when they would all live in Eretz Yisrael the Leviim would become an unbearable burden upon the other tribes, as they are involved in serving HaShem and are supported by everyone else. As well, more or larger parcels of land would have to be cut out of the other tribes' inheritance to supply them with land for housing. (Meshech Chochmah) This opinion is mentioned in the Kli Yakar and he resoundingly disagrees with it.
9. Whatever is more dear, its development is more complicated and more difficult to come to fruition. The tribe of Levi, the spiritually chosen of HaShem, were therefore a limited group. (Haameik Davar)
10. Rashi on Bemidbar 26,5 says that the Canaanites claimed that the Bnei Yisrael were not of pure stock, reasoning that if the Egyptians were masters over the men, surely, they lorded over their wives. The extreme population explosion of the Bnei Yisrael clearly counters this claim since the Gemara Avodah Zarah 54b says that if one were to commit adultery with a woman, by right the woman should not get pregnant, as a sin was committed. However, HaShem allowed the world to proceed by the "laws of nature" and it is possible to become pregnant even through an act of sin. This reasoning preempts a population explosion brought about by adultery. It is with difficulty that HaShem allows a pregnancy, but He would not allow multitudes beyond the "laws of nature" to be conceived in sin. Therefore, the population explosion of the Bnei Yisrael counters the claim that the Egyptians sinned with the Bnos Yisrael.

This is an insight into "V'chaa'sher y'anu oso kein yirbeh" (Shmos 1,12). The more the Egyptians pained the Bnei Yisrael and lorded over them, the more the Bnei Yisrael multiplied, testimony that the Bnei Yisrael were not sired by the Egyptians. Since the Bnei Levi were not enslaved by the Egyptians, as per M.R. Shmos 5:16, there never was a claim that their wives were misused by the Egyptians. Since there was no such claim there was no need for the miracle of extreme population growth. This is also why the Leviim were counted separately (1,49), as they needed no proof for their pure lineage, as stated, "V'haLeviim l'ma'tei avosom" (1,47). (Adapted from the words of the Sha"ch and M'lo Ha'omer)

PLEASE NOTE: It is worthwhile to see many of the above commentaries at the source, as they fill in more detail. As well, we find in Divrei HaYamim that the Leviim increased in number in Eretz Yisrael more than any other tribe. According to some of the above-mentioned answers this is most difficult to explain.